There are two types of people in this world. Those who use the flash on their phone and those who don't. I fall in the latter category and do everything in my power to avoid using my phone's flash.
A flash is distracting, and can make subjects look like a glowing white light zombie standing against a dark background. Flashes normally use a higher shutter speed, which in a low-light situation causes the zombie on a dark background effect.
Basically the iPhone takes a photo at a slower shutter speed while firing the flash quickly. With the shutter open longer, the background is exposed more making it brighter. And with the flash going off for a shorter amount of time, it's less distracting to those around you. The results are to quote Larry David, "pretty pretty pretty good."
Check out the photos below taken with the flash inside a bar. The one from the 7 Plus ($672.38 at Amazon.com) has a darker background and lots of nasty harsh light on the subject. There's even a little red-eye happening. The 8 Plus blends the light from the flash with the background. The overall image looks good.
Slow Sync Flash is definitely my favorite feature on the new iPhones even more so than Portrait Lighting Mode. And the best part? You don't have to anything but enable your flash. The iPhone takes care of the rest.
See more from our iPhone 8 Plus vs. iPhone 7 Plus camera testing here.